Intrinsic modules

Last updated on 2026-07-10 | Edit this page

Estimated time: 20 minutes

Overview

Questions

  • What intrinsic modules does Fortran provide?
  • How can intrinsic modules aid writing portable code?

Objectives

  • Be aware of the intrinsic modules in Fortran and their uses

Intrisic and non-intrinsic modules

Five intrinsic modules are provided by the implementation: we have seen iso_fortran_env.

Three are related to IEEE arithmetic: ieee_arithmetic, ieee_exceptions and ieee_features. The last is iso_c_binding which concerns C/Fortran interoperability.

A use statement may include, optionally, an intrinsic or non_intrinsic attribute, e.g.,

  use, intrinsic :: iso_fortran_env

This true intrinsic iso_fortran_env will then be used in preference to any other module with the same name.

Likewise a non-intrinsic module can be used:

  use, non_intrinsic :: iso_fortran_env

This would generate an error if a user-provided iso_fortran_env were unavailable to the compiler.

Best practice is to always use an only clause with intrinsic modules, which enumerates the names required in the current scope. This is a general rule which limits “namespace pollution”, and can be applied to your own modules too.

Intrinsic module iso_fortran_env

This module provides a number of symbolic constants and functions to provide information about the local Fortran implementation.

These include:

  1. error_unit, input_unit and output_unit: the unit numbers associated with standard error, standard input, and standard output.
  2. iostat_end and iostat_eor: error codes for end-fo-file and end-of-record.
  3. character_storage_size, file_storage_size, and numeric_storage_size related to i/o

F2008 included kind type parameters for frequently used intrinsic data types:

  1. int8, int16, int32, int64: integer kind type parameter for intrinsic integers (in bits);
  2. real32, real64, real128: integer kind type parameters for intrinsic real data types.

Lists of the kind types available for each intrinsic data type are also available: integer_kinds, logical_kinds, real_kinds, character_kinds.

Functions

Information on the compiler and the options at compile time can be accessed via the functions: compiler_version() and compiler_options(). Both return a string fixed at compile time.

E.g.,

  character (len = *), parameter :: compiler = compiler_version()

Example (3 minutes)

Discussion

iso_fortran_env values

An example program is provided which prints out the values of the various symbols available from iso_fortran_env.

$ ftn print_iso_fortran_env.f90

To see the actual values in the kind arrays (integer_kinds and so on) some extra statements would be required.

IEEE arithmetic support

Fortran provides support for programmers to inquire about support for various aspects of the IEEE standard for floating point arithmetic. This information is separated into three modules.

  1. ieee_exceptions provides handling for IEEE exception types enumerated by the values: ieee_overflow, ieee_divide_by_zero, ieee_invalid, ieee_underflow and ieee_inexact.

  2. ieee_arithmetic contains functionality for identifying different classes of floating point values (e.g., ieee_negative_inf), rounding modes, and so on. The ieee_arithmetic module include ieee_exceptions.

  3. ieee_features does not provide names a such by may affect compilation if present. E.g., if

      use, intrinsic :: ieee_features, only : ieee_subnormal

    is present, then the program unit must provide support for subnormal floating point numbers (aka denormalised numbers, with a leading zero in the mantissa).

A full description of the IEEE features will not be attempted here.

Key Points
  • The iso_fortran_env intrinsic module provides constants and functions that enable code portability
  • The IEEE intrinsic modules allow programs to check the conformance with IEEE features